A nickel-metal hydride storage battery, a nickel cadmium storage battery, a nickel zinc storage battery, and the like have been conventionally used as alkaline storage batteries. A nickel-metal hydride storage battery that has high capacity and is excellent in environmental safety has recently become widely used as a power source for an electric vehicle, a hybrid vehicle, an electric motorcycle, electric tools, and the like.
In such nickel-metal hydride storage batteries, an alkaline electrolyte is employed and a separator is provided between a positive electrode which includes nickel hydroxide and a negative electrode which includes a hydrogen-absorbing alloy.
As the separator, a woven or nonwoven fabric comprising polyamide and polyolefin, or a porous film of a fluorine plastic film is generally used.
In the above described nickel-metal hydride storage battery, a nitrogen-containing ion, for example, ammonium ion, nitrate ion, or the like, is included as an impurity in a positive electrode, etc. The nitrogen-containing ion causes self-discharge in the nickel-metal hydride storage battery and inferior storage characteristics of the battery.
A separator which is an acrylic monomer graft-polymerized onto a polyolefin material has been recently proposed to solve such problem. A carboxyl group included in the separator traps a nitrogen-containing ion impurity. A separator in which a sulfo group has been introduced into a polyolefin material by treatment of the polyolefin material with concentrated sulfuric acid or fuming sulfuric acid has also been proposed. In the separator, a sulfo group traps nitrogen-containing ions.
However, a separator material made of an acrylic monomer graft-polymerized onto a polyolefin generally has poor heat resistance and acid resistance. Therefore, it is difficult to use it for an extended period and the life of the nickel-metal hydride storage battery is shortened.
The separator in which a sulfo group is introduced into a polyolefin material by treatment of the polyolefin material with concentrated sulfuric acid or fuming sulfuric acid has a problem in that the separator is deteriorated by the treatment with a concentrated sulfuric acid or fuming sulfuric acid and likely causes a short circuit between positive and negative electrodes.
Recently, studies have been carried out to make a separator thin to increase capacity and power output of a nickel-metal hydride storage battery.
However, a hydrogen-absorbing alloy used for a negative electrode is pulverized and separates from the negative electrode after repeated charging and discharging of a nickel-metal hydride storage battery. When the separator is made thinner, fine powder of the hydrogen-absorbing alloy passes through the separator, and causes a short circuit to reduce the capacity of the battery. Especially, when the battery is used as a power source for an electric vehicle, a hybrid vehicle or an electric motorcycle, an amount of fine powder of the hydrogen-absorbing alloy which separates from the negative electrode increases due to vibrations, and a short circuit is more likely to occur to reduce a capacity of the battery.
Therefore, a separator which includes a laminate of a woven or a nonwoven alkaline resistant fabric and a fine porous fluorine plastic film is disclosed in Japanese Patent Laid-open Publication No. 9-330692.
However, the separator described above does not have a sufficient hydrophilic nature because of the use of the fluorine plastic film. Charge and discharge characteristics of the battery are diminished and cost of the battery is also high.